Implantable medical devices (IMDs) provide therapies and monitor a wide variety of physiological events. With the increased uses of IMDs has also come the need for improved methods of communicating with and between IMDs.
Conventionally, communication with IMDs has been with magnetic field communication systems. Such systems, however, are generally only capable of communicating over very short distances, on the order of a few inches. As a result, a magnetic head of a programmer (or other external device) needs to be placed near to the IMD for communication to occur. More recently, radio frequency (RF) based communication systems have been developed for use with IMDs. RF communication provides a number of benefits over magnetic field communication systems, including much greater communication distances. However, conventional RF communication systems consume more battery power than magnetic field communication systems, thus impacting the service life of the IMD battery.
Accordingly, there is a need to improve RF receiver efficiency and inter-IMD communication modalities to conserve battery life.
RF communication may generally be divided into two categories: synchronous and asynchronous. Synchronous communication is conducted at scheduled times. However, in synchronous communication systems, the internal clocks of two communicating devices are prone to drift over time. As more time elapses, the internal clocks become increasingly out of sync, such that neither device can precisely detect when the other device will commence communication. To compensate for this drift, one or both of the devices must stay in an “on” mode. During that time, energy is consumed while no communication is effected.
In an asynchronous communication system, transmission occurs at random times. Because it is impractical to maintain the receiver on at all times, asynchronous communication systems utilize sampling methods in which the receiver is repeatedly turned on for brief periods to check for a transmission signal and turned on fully when the signal is detected. The more often the receiver is turned on, the faster the response time of the receiver. However, more energy is required. To guarantee that data will be received, the transmitter transmits a preamble for at least as long as the time interval between samples prior to transmitting a message. Once the preamble is detected, the receiver remains on until the message is received. As a result, energy is consumed by the receiver while receiving the preamble, a time in which no valuable communication is taking place.